Increased levels of atmospheric carbon dioxide may have contributed to a gradual greening of some desert regions over the past 30 years, a process that will continue, according to a new study. But the authors warn that this "CO2 fertilization effect" could also have consequences for native plants and the wildlife that depends on them.
The study, published May 15 in the journal Geophysical Research Letters, was conducted by researchers from the Commonwealth Scientific and Industrial Research Organization (CSIRO), Australian National University and the Australian Research Council Centre of Excellence for Climate System Science.
The researchers went into this project knowing that satellite data collected since the 1980s has shown a worldwide increase in green foliage. Scientists around the world have theorized that this may have been a result of increased levels of CO2, a theory this new study supports. The authors of this new study looked at desert areas on four continents — where increases in vegetation would be easier to see and quantify — and created a mathematical model to calculate how they might have been affected by CO2 fertilization. With those calculations in hand, they then compared their predications to satellite imagery data from 1982 to 2010.
Knowing that CO2 levels have increased 14% during that period, the researchers calculated that desert foliage would have increased from 5-10% during that 28-year period. The satellite data revealed that they average increase in foliage during that time was 11% (a number that was adjusted for short-term precipitation changes). The researchers call this correlation "strong support for our hypothesis," although it is not conclusive proof.
"Lots of papers have shown an average increase in vegetation across the globe, and there is a lot of speculation about what's causing that," lead author Randall Donohue of CSIRO said in a news release about the study. "Up until this point, they've linked the greening to fairly obvious climatic variables, such as a rise in temperature where it is normally cold or a rise in rainfall where it is normally dry. Lots of those papers speculated about the CO2 effect, but it has been very difficult to prove."
The direct link of this greening to CO2 has remained a theory for because, as Donohue explained to NBC News, "There are so many processes occurring simultaneously that affect plant behavior, it is very difficult to determine which process is responsible for any given change."
The researchers warn that CO2 fertilization could have negative effects for native plants in these desert regions. "Trees are re-invading grass lands, and this could quite possibly be related to the CO2 effect," Donohue said. "Long-lived woody plants are deep rooted and are likely to benefit more than grasses from an increase in CO2." Increased tree levels in arid regions could, meanwhile, increase the threat of forest fires.
Donohue said the effect of increased carbon dioxide levels on plants should be a greater focus of global study. "It needs to be considered as an important piece of the overall global-change puzzle that we are still trying to figure out," he told NBC.
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